A ring oscillator may include an odd number of inverters that are configured to generate an oscillating output signal that oscillates between two voltage levels. The inverters may be chained together such that the output of one of the inverters is connected to the input of another of the inverters. Under the fundamental law of oscillation, the chain of inverters may provide a loop gain higher than one and total phase shift of 180 degrees in order to generate oscillating output signal.
Ring oscillators may be used in a system where a frequency of the oscillating output signal can be used to determine a process-voltage-temperature characteristic of the system. In one example, a ring oscillator may be used as a process detector. Due to imperfections in the fabrication process, transistors of a same type that are located in different areas of a same die, or located on different dies may have different speeds. After fabrication, dies may be tested and categorized as typical, fast, or slow in order to further calibrate the dies so that their components operate at desired speeds. A frequency of the ring oscillator output signal may be used to identify a die or a particular set of transistors on the die may as being in the typical, fast, or slow category. In another example, a ring oscillator may be used as a temperature detector that detects an operating temperature of a system. An approximately linear relationship may exist between the frequency of the ring oscillator output signal and temperature. Based on the relationship, a change in the frequency may indicate a change in the temperature of the system.
In addition to temperature, the frequency of the ring oscillator output signal may also be affected by noise in a system, such as noise on a supply voltage line. The greater the amplitude of the noise, the greater that the frequency of the ring oscillator output signal may vary. If the noise amplitude is too large, then the variation of the frequency due to the noise may mask the variation of the frequency due to a change in temperature. In other words, if the noise is too large, then the ring oscillator may generate its output signal at a frequency that does not accurately reflect the operating temperature of the system. As such, it may be desirable to reduce the impact that noise has on the ring oscillator so that the ring oscillator may generate its output signal at a frequency that most accurately reflects the operating temperature of the system.